The invention disclosed herein is generally related to high frequency induction plasma tubes and, more specifically, to induction plasma tubes having internal radiation shields.
High frequency induction plasma tubes are well-known for producing high temperature gaseous plasmas. Such plasmas are useful in a number of practical applications, including high temperature spectroscopic studies and the preparation of microcrystalline refractory materials.
An induction plasma tube consists essentially of an electrical induction coil surrounding an enclosure which contains an ionizable gas. The coil is connected to a source of high frequency (400 kHz to 5 mHz) electrical current. The enclosure typically consists of a quartz tube centered inside the coil. Argon is a commonly used ionizable gas. Upon application of power to the induction coil the gas is ionized, producing a central core of hot gaseous plasma inside the enclosure.
At low power levels the plasma is concentrated toward the center of the enclosure such that there is no danger of heat damage to the enclosure walls. At high power levels, however, the plasma core is both hotter and larger in diameter. As a result, the quartz enclosure is easily damaged by the plasma, which typically attains temperatures on the order of 10,000.degree. C. and above. This problem is aggravated by the fact that the plasma is typically subject to magnetic and electric instabilities that cause it to fluctuate in position and occasionally contact the enclosure walls. High power levels also result in the emission of intense ultraviolet radiation from the plasma, which ionizes the air around the enclosure and results in electrical arcing in the induction coil. These adverse effects have lead to the use of internal water-cooled radiation shields, located inside the enclosure, to protect the enclosure walls and block emission of ultraviolet radiation from the plasma core. Such shields are commonly used in addition to other protective cooling measures, for example the use of double-walled water-cooled enclosures and the use of a continuously flowing stream of coolant gas along the inside surface of the enclosure.
The previously known internal shields are tubular in shape, thin-walled, and are sized slightly smaller in diameter than the tubular quartz enclosure so as to fit closely inside the enclosure and surround the plasma core. Such shields have typically been formed of thin copper tubing through which coolant water is pumped. For example, one prior art shield consists of multiple hairpin-shaped coolant tubes which extend axially into the quartz enclosure from a manifold. Water is pumped from the manifold down one side of each tube and returns upwardly through the other side to a water return duct in the manifold. One disadvantage of this design is that the return side of each tube is always warmer than the supply side, since the coolant water is progressively warmed as it travels through the tube. As a result of this uneven cooling and the thin-walled construction, the shield is easily damaged by the plasma and does not adequately protect the enclosure walls.
The radiation shield must function as a barrier to a substantial portion of the heat and radiation emitted from the plasma, yet at the same time it must be transparent to the electric and magnetic fields produced by the coil. The latter requirement has previously been assumed to have been met, according to considerations based on conventional electromagnetic theory of induction plasma tubes, by making the shield as thin as possible and by utilizing a segmented construction. For example, the above-mentioned prior art shield is formed of thin-walled, small diameter copper tubing, with the individual coolant tubes being spaced circumferentially from one another. As discussed further below, it has now been found that this assumption is incorrect, and that there are in fact advantages to using a thick-walled, segmented construction.
With a prior art shield of the type described above, maximum attainable plasma temperatures have been limited to approximately 18,000.degree. C. However, such temperatures have only been attainable by maintaining a relatively high flow rate of gas through the enclosure to assist in cooling the shield and the enclosure. The turbulence resulting from this gas flow has several disadvantages. For example, in the preparation of microcrystalline refractory materials such turbulence results in a less uniform particle size distribution, and in spectroscopic studies it results in broadened peaks and spurious signals. Additionally, turbulence contributes to instability in the plasma arc itself, which frequently makes it difficult to initiate and sustain the plasma over a period of time.